1. Field of the Invention
In the context of insulated gate field effect transistors, the invention relates generally to the field of integrated circuit devices and more particularly to the structure of integrated circuit devices.
2. Background Information
The use of metal gate technology is viewed as very desirable for complementary metal oxide semiconductor (CMOS) device technology scaling below the sub 0.1 micron regime. Replacing traditional polysilicon gate electrodes with metal or metal alloy gate electrodes may reduce undesired voltage drops associated with polysilicon gate electrodes (e.g., polysilicon depletion effect) and improve device drive current performance. Metal and metal alloy gate electrodes may also reduce the parasitic resistance of the gate line and allow longer gate runners in high performance integrated circuit design for applications such as stacked gates, wordlines, buffer drivers, etc. Longer gate electrodes correspond to field effect transistors of greater width.
Conductive materials have different energies measured conventionally by their Fermi level. As an example, the Fermi level of a material determines its work function. The intrinsic Fermi level of an undoped semiconductor is at the middle of the bandgap between the conduction and valence band edges. In an N-type doped silicon, the Fermi level is closer to the conduction band than to the valence band (e.g., about 4.15 electron-volts). In a P-type doped silicon, the Fermi level is closer to the valence band than the conduction band (e.g., about 5.2 electron-volts).
Metals or their compounds have been identified that have work functions similar to the work functions of a conventional P-type doped semiconductor substrate. Other metals or their compounds have been identified that have work functions similar to a conventional N-type doped semiconductor substrate. Examples of metals that have a work function similar to P-type doped semiconductor material, include but are not limited to, nickel (Ni), ruthenium oxide (RuO), molybdenum nitride (MoN), tantalum nitride (TaN), molybdenum silicide (MoSi.sub.2), and tantalum silicide (TaSi.sub.2). Examples of metals that have a work function to N-type doped semiconductor material, include but are not limited to, ruthenium (Ru), zirconium (Zr), niobium (Nb), and tantalum (Ta).
Previously proposed metal gate CMOS technology has focused on using one type of metal having a Fermi level located in the middle of the conduction and valence band of the silicon substrate (e.g., work function of about 4.7 electron-volts). One drawback of mid-bandgap metals, however, is their inability to achieve the small threshold voltage (V.sub.T) desirable for future CMOS technology scaling, without degrading short channel effects.
A complementary metal gate approach with two work functions, optimized for both NMOS and PMOS devices, respectively, thus far has yet to be integrated into a workable process. The simple method to deposit complementary metals, one after the other, would damage the thin gate dielectric during patterning of at least one of the electrodes making the transistor unusable.
What is needed is the incorporation of complementary metal gate electrode technology into a workable process that is scalable for future CMOS technologies.